As we know that most of the computers are effected due to viruses. And we also know that USB is also one of the source that mostly contains viruses as it is portable device that is used to move data from one location to other or for backup purpose. In this way if some viruses are there in the network so it become the source to move the viruses to other network or stand alone pc.
To make your system/Network safe one have to follow the steps to enable or disable usb on system.

- Go to run and write "reg edit"
- Click on "HKEY_LOCAL_MACHINE"
- System
- CurrentControlSet
- Services
- USBSTOR
then Double click on start and change the value to 4 for usb blocking or 3 for usb opening.